Such a transformer-testing apparatus typically has a static frequency converter that includes multiple outputs connected to a filter. The filter also has multiple outputs that are connected to a matching transformer connected to the transformer being tested.
An article titled “High-Performance Digital Control of Uninterruptible Power Supply (UPS) Using an Applications-Specific Integrated Circuit (ASIC) was published in the conference proceedings of the “European Conference on Power Electronics and Applications.” The conference took place 3-6 Sep. 1991 in Florence. The conference proceedings were also published in 1991. For the purpose of complete digital control of a UPS system, an appropriate ASIC was designed that provides fast current control and voltage control. Use of this ASIC with the UPS system enables excellent dynamic response to be achieved (rapid step response, low harmonic distortion with nonlinear loads). The digital circuit comprises seven PI controllers, a sine-wave generator, an interface to the analog-to-digital converters, and PWM generation for the switching signals. An uninterruptible power supply (UPS) is required for all applications where an electrical system must operate even in the event of a power failure.
German patent publication DE 19 239 85 [U.S. Pat. No. 3,555,291] describes a filter for a power-transmission system. In AC systems, it is often necessary to filter out unwanted harmonic oscillations. This multiphase electrical power transmission system is composed of multiple conductors that have a certain inductance. An alternating current is applied to the conductors, which current has both typical and atypical harmonic components. In addition, multiple filters for typical harmonic components are connected to the conductors. In addition, another filter is provided with an impedance that is coupled to the conductors and functions to attenuate the atypical harmonic components of the system voltage. This typical harmonic component is created by the interaction between the atypical harmonic component of the current at high impedance that results when the inductance and the impedance of the harmonic filter are in resonance. However, this document does not mention anything about the testing of transformers.
Published German patent DE 42 323 56 discloses a power supply unit comprising at least two current sources. Each current source is fed by at least one respective power converter into an AC system. In order to reduce the amplitude of a given harmonic component that is present, the voltage of at least one power converter is fed into the system with a trigger delay angle when phase-shifted relative to at least one other power converter.
Published German patent DE 19 546 420 [U.S. Pat. No. 5,949,662] also discloses an uninterruptible power supply. The uninterruptible power supply comprises a line-side commutation reactor, a voltage source inverter with associated controller, an output filter composed of a transformer of large leakage inductance and filter capacitors to the static bypass switch with associated controller, and a manual bypass switch. A frequency converter from the field of drive engineering is used as the converter. An input filter is connected on the input side of this frequency converter, a battery connection device being provided at the DC terminals. As a result, it is possible to construct an uninterruptible power supply using commercially available components from the field of drive engineering and with UPS-specific components, which supply utilizes the technical and economic synergies of drive engineering. The document also does not deal with the testing of transformers.
EP 0 268 046 describes a method of and a circuit for regulating and controlling three-phase inverters with sequence regulation. The focus of this application is on describing a system subject to an asymmetric load. The use of a transformer between the inverter and filter unit is listed as an example of an application. The transformer functions only to convert voltage.
DE 26 08 541 [U.S. Pat. No. 3,963,975] describes an electromagnetically shielded power supply in which an electrical cancelling signal is obtained on the secondary side of the transformer in addition to the electrostatic screen for attenuating asymmetrical disturbance variables, the signal being fed in phase opposition into the useful-signal current circuit.